This is a concern of mine too ever since about two years ago when a local woman was raped and killed by a cab driver in San Francisco. AnnaS, I believe, also posted about a woman who was attacked by a shuttle driver.

Punki had a good suggestion when this was discussed on an old thread. She recommended using your cellphone and having a conversation with your husband, whether you have one or not and whether you actually reach him or not. As I recall her cellphone conversation went something like:

"Hello dear, yes, I'm on the way to the hotel now. This wonderful driver - what's your name? - yes, my wonderful driver Fred is just now turning on to Jefferson Street at Jackson so we should be there in about 15 minutes ... "

At least the driver would know he'd get caught pretty quickly if he tried anything.